Revert 153092 - Turn on warnings as errors for (most) third_party code on Windows

Is causing warnings for Official build in PDF code. Boo.

-----------------

Can't quite be fully enabled yet due to not-yet-fixed third party
dependencies. Without it enabled, other packages regress while we're fixing
things. So, add a flag for now so warnings-freeness can be ratcheted forward
by having it on for most packages, but off for a few.

Also, disable warning in qcms (fixed upstream by a large refactoring, not
worth rolling for), and disable two minor warnings in yasm (patch posted
upstream for a few months, but maintainer does not seem motivated to merge).

Fix release-only warning in leveldatabase/env_chromium.cc. Was calling exit(1)
in a leaky destructor.

Fix a warning in lzma_sdk (missing an include).

Disable a silly warning in Release builds of skia and memory_watcher (that
/GS is not working because optimization is disabled).

Warning are currently tolerated in libvpx and mesa.

Cannot be committed until http://chromiumcodereview.appspot.com/10823426/ has
landed.

R=thakis@chromium.org
BUG=126483,140121, 143877
TEST=


Review URL: https://chromiumcodereview.appspot.com/10827429

TBR=scottmg@chromium.org
Review URL: https://chromiumcodereview.appspot.com/10872063

Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 225507326ae5af8f9cf644aaee91174bd0e3602d
diff --git a/yasm.gyp b/yasm.gyp
index 9093600..d981b50 100644
--- a/yasm.gyp
+++ b/yasm.gyp
@@ -71,9 +71,6 @@
         'genstring',
         're2c',
       ],
-      # Patch to fix posted upstream, but not yet merged.
-      # https://github.com/yasm/yasm/pull/64
-      'msvs_disabled_warnings': [ 4018, 4090 ],
       'sources': [
          'source/patched-yasm/frontends/yasm/yasm-options.c',
          'source/patched-yasm/frontends/yasm/yasm.c',